Add cookie domain configuration under system.session

[MaxEvan]

I have added a new config under "system.session" called "domain".

The current implementation is

$domain = $uri->host();
if ($domain === 'localhost') {
    $domain = '';
}

This is problematic under docker-compose because the hostname is the name of the container/service. In our case, it was forcing the cookie domain to be "domain=grav-site;".

Basically, under docker, the only way to make it work would be to use "localhost", but that would break a lot of stuff.

This setting allows override of the domain, and it shouldn't break existing applications.

[mahagr]

This change will break multisite setups and may have side effects on sites without session domain being set.

[mahagr]

Implemented in above commit.